In the computer field for example, there is a need to provide filtration within the sealed enclosures of hard or "Winchester" disk drives. Hard disk drives have an inflexible platter coated with magnetic material which is spun very rapidly. A magnetic read/write head "flies" only a few microns above the disk on an air cushion. If particulate matter were to become lodged between the disk and the head, the disk drive would be destroyed. On the other hand, there is no space for large or sophisticated filtration equipment within the drive since it is a requirement to make the drives as small as possible.
The two requirements of high efficiency and limited space tend to conflict; therefore, special techniques are required to produce a filter assembly. A pleated filter media which maintains proper pleat spacing, maximizing capacity of the filter and also exhibits minimum loss of media fiber is shown in U.S. Pat. No. 4,617,122 to Donaldson Company, the assignee hereof.
The predominant apparatus for making pleated filter media is the rotary type pleater. The rotary type apparatus pleats media by passing a sheet of filter paper between a pair of rollers including a plurality of scoring tools having scoring blades or knifes at the outer periphery thereof adjacent the outer periphery of the rollers. The scoring blades irreversibly indent or crease the media forming lines along which the media is folded to form pleats. Other known pleating devices also score the filter media during formation of the pleated media.
While the scoring process is acceptable for forming pleated filter media useful in some applications, scoring produces a number of undesirable drawbacks when relied on to form pleated media for certain high efficiency applications. For example, scoring fractures or breaks the continuous fiber structure of the filter media creating areas where undesirable impurities can leak through the media. In many cases, a protective layer of synthetic material such as polyester or nylon (i.e. scrim) is laid over the media to compensate, in part, for the damage to the media caused by the scoring process. The inclusion of scrim on the media surface; however, requires additional handling steps and increases costs.
Accordingly, there is a need for an apparatus and method which can produce a pleated filter assembly designed for use in environments requiring high efficiency filtration. The present invention meets this need by providing a filter assembly including filter media exhibiting minimal disruption of its fiber structure. The apparatus and method of the present invention also require a minimum number of steps to produce a complete filter assembly; thereby minimizing damage and contamination to the media resulting from multiple handling of the media during production.